re: frugal ac tips
4 aug 2002
>>if nick's house was an automobile...
but it isn't.
>>and his ac was the engine, his clogged filter would be analogous to
>>a clogged fuel filter... it puts more strain on his fuel pump and
>>de-rates the performance of his engine.
>where is the "strain" in this case?
here's a quote from kreider and rabl's book heating and cooling of
buildings (mcgraw hill, 1994):
the performance of air-cooled condensers can be improved if the
air-side surface is kept wet with purified water. evaporation from
the condenser will enhance performance markedly because the driving
potential for a coil cooled by evaporation is the wet-bulb, not the
dry-bulb, temperature. since the wet-bulb temperature is 15 to 25 f
(8 to 14 c) below the dry-bulb temperature, evaporative condensers
will operate at temperatures substantially lower than those of
air-cooled equipment and somewhat lower than those of water cooled
condensers employing a cooling tower. low condenser temperatures
result in lower compressor power needs and longer compressor life.
the key consideration in the design of evaporative condensers is
the water composition. if minerals are not controlled, they will
accumulate on the condenser sufrace and foul it, reducing heat
transferability in relatively short order. biological growth can
also foul the surface. the cost of water must also be considered
in arid climates.
goswami's 1993 asme paper begins:
the concept of using evaporative cooling is widely employed in
cooling towers for air conditioning systems for large facilities
and to provide cooling for agricultural greenhouses. it is a proven
concept (ashrae, 1989) but is used on a very limited basis for
residential and small commercial air conditioning systems, which
are usually air-to-air compression-type systems.
the performance of such systems can be improved by employing
indirect evaporative cooling such that no moisture is added
to the supply air stream. this can be achieved by evaporative
cooling of the outside air before it passes over the condenser
coil, resulting in a larger overall temperature difference
across the heat exchanger and thus greater cooling effect.
the process requires very little additional energy input for
the condenser fan and for water pumping, resulting in higher
coefficient of performance. increased cooling effect allows
the equipment to be downsized resulting in lower peak demand
and energy usage. this can result in significant energy and demand
savings overall since there are millions of air conditioners in
the residential sector that can be easily retrofitted. a simple
retrofit on the condenser of an air conditioning unit can save
enough energy to pay for itself in as little as one to two years.
indirect evaporative cooling has rarely been used for residential
and small commercial air-to-air vapor compression air conditioning
systems due to the following reasons:
(1) spray of water on the coil causes mineral deposits on the heat
transfer surfaces which decreases the performance of the system, and
(2) spray of water on the coil makes it essential to have an
expensive water treatment system to prevent scale buildup on
the heat transfer surfaces.
the above problem can be eliminated if an alternative system, eg a
media pad evaporative cooler is used. media pads are cellulose-bound
cardboard structures which are cross-fluted to increase the transfer
area. the average life expectancy of a media pad evaporative cooler
is of the order of five years. with clean water, it may last up to
ten years (munters, 1986).
with this system, air is allowed to pass through the wetted medium
(media pad) before entering the condenser. hence water droplets do not
reach the heat transfer surfaces of the condenser coil, thus avoiding
mineral deposits on the coil. the mineral deposits occur on the
inexpensive media pads, which can be replaced periodically as needed.
the present investigation was conducted to demonstrate the practicality
of this concept for new air conditioning units as well as retrofits of
existing units and to evaluate actual savings employing this concept.
this paper presents the results of performance improvement of
a residential air conditioner retrofitted with a media pad cooler.
goswami et al built a custom sheet metal swamp cooler (with pads on 4 sides
and a water pump but no fan) around the standard outdoor condensing unit of
a 2.5 ton (30k btu/h) trane heat pump attached to an occupied test house.
the airflow rate was 2430 cfm, the pads were 8" thick, and the pad face
velocity was 195 to 212 fpm. they might have built their swamp cooler box
from a standard aluminum greenhouse swamp thing (tm) or kool-cel (tm) kit
from a supplier like stuppy at (800) 733-5025.
they measured electrical power with individual meters on the compressor,
the fans and the pump, and measured a 0.025" wc pressure drop across the
pads with an inclined manometer. they also measured outdoor conditions,
refrigerant pressure at various locations, and air and refrigerant temps.
on a typical day, they measured 2.9-3.0 kw of compressor power without
and 2.4 kw with evaporative cooling, with an eer increase from 9 to 11,
ie a 22% system performance increase.
the average air precooling was 14 f. the worst-case water evaporation
rate was 8.74 gallons per hour. at $0.085/kwh and $0.70/1000 gallons of
water, they estimate the cost of running the pump at $6.64/year, the cost
of additional fan power at $2.36/year, and the cost of water at $15.42/year.
on a smaller and simpler scale, a window unit might have condensation
and rainwater from a downspout trickling over condenser fins, with a
12w $20 bilge pump in a recycled plastic 55 gallon drum below.
why don't people do this more? many might say "why bother?" given today's
us energy prices, maintained artificially low by taxpayer-funded wars.